1. Field of the Invention
The present invention relates to an electrooptical apparatus having a function causing a part of a display screen to be in a display state and causing the other to be in a non-display state and a driving method therefore. Furthermore, the invention, using a liquid crystal display apparatus as the electrooptical apparatus, relates to the driving method for the liquid crystal display apparatus, which allows a partial display state without providing an incompatibility and with less power consumption, and it also relates to the liquid crystal display apparatus performing display operation according to the above. The present invention also relates to a driving circuit suitable for driving the electrooptical apparatus of the invention.
Furthermore, this invention relates to an electronic equipment to be used for the electrooptical apparatus and the display apparatus described above.
2. Description of Related Art
With display apparatuses being used for portable electronic equipments such as portable telephones, the number of display dots is increasing year by year so that increasing amounts of information can be displayed. Accordingly, power consumption by the display apparatus is also increasing. Generally, the portable type electronic equipment uses battery as a power source; therefore, reduced power consumption with the display apparatus is strongly demanded so that battery service life can be extended. That is why, a study has begun for development such that with a display apparatus having a larger number of the display dots, a full screen is displayed when it is necessary; however, in normal use, only a partial region of a display panel is allowed to be in a display state and the other is left in a non-display state so that power consumption can be reduced. Furthermore, in response to the demand for power-consumption reduction, as display apparatuses of portable type electronic equipment, liquid crystal display panels of a reflective type or a transflective type designed by placing importance on appearance in a reflection mode is used.
In conventional liquid crystal display apparatuses, they have, in most cases, a function allowing control of display/non-display operations on a full-screen basis; however, a display apparatus having a function that allows only part of a full screen to be in a display state and allows the other to be in a non-display state has not been realized to date. A method to realize a function that allows only partial lines of a liquid crystal display panel to be in a display state and the other to be in a non-display state has been proposed with Japanese Unexamined Patent Publication Nos. 6-95621 and 7-281632. Both of these two proposals disclose a method in which display duties are varied according to the case of a partial display and the case of a full-screen display so as to obtain driving voltages and bias ratios which are suitable to the individual duties.
The method proposed in Japanese Unexamined Patent Publication No. 6-95621 will be described below with reference to FIGS. 19 to 21. FIG. 19 is a block diagram showing an example of conventional liquid crystal display apparatuses. A block 51 represents a liquid crystal display panel (LCD panel) in which a substrate on which plural scanning electrodes are formed and a substrate on which plural signal electrodes are formed are arranged to oppose each other with a several-μm gap, and a liquid crystal is enclosed in the gap. By the liquid crystal at cross sections of the scanning electrodes arranged in the line direction and the signal electrodes arranged in the column direction, pixels (dots) are to be formed in a matrix. A block 52 represents a scanning-electrode driving circuit (Y driver) that drives the scanning electrodes, and a block 53 represents a signal-electrode driving circuit (X driver) that drives the signal electrodes. Plural voltage levels necessary for driving the liquid crystal are formed in a driving-voltage forming circuit represented by a block 54 and are applied to the liquid crystal display panel 51 through the X driver 53 and the Y driver 52. A block 57 represents a scanning control circuit that controls the number of the scanning electrodes to be scanned. A block 55 represents a controller that supplies signals necessary for these circuits, FRM denotes a frame start signal, CLY denotes a scanning-signal transfer clock, CLX denotes a data transfer clock, Data denotes display data, LP denotes a data latch signal, and PD denotes a partial display control signal. A block 56 represents a power source for the circuits described above.
In this conventional example, a case in which the partial display appears on the left-half screen and on the upper-half screen is described; however, hereinbelow, a description will be given of the latter case in which lines for the upper-half screen are arranged in the display state and lines for the lower-half are arranged in the non-display state. The number of the scanning electrodes is assumed to be 400. The controller 55 turns the partial display control signal PD to an H level to allow the lower-half screen to be in the display state. When the partial display control signal PD is at an L level, all the scanning electrodes are scanned at a 1/400 duty, by which the full-screen is turned to the display state. When the partial display control signal PD is at the H level, only the scanning electrodes for the upper-half screen are scanned at a 1/200 duty, by which the upper-half screen is turned to the display state and the remaining lower-half screen is turned to the non-display state. Switching to the 1/200 duty is performed by switching to the duplicated cycle of the scanning-signal transfer clock CLY to reduce the number of clocks in one frame period. A scanning-stopping manner for the scanning electrodes for the lower-half screen in the partial display state is not described in detail. From the internal circuit diagram of the scanning control circuit block 57, however, the manner is considered to he such that as follows. That is, when the control signal PD is turned to the H level, data to be transferred from the 200th stage to the 201st stage of a shift register in the Y driver is fixed at the L level, resulting in that outputs of the 201st to the 400th from the Y driver, which are fed to the scanning electrodes of the 200th to the 400th, are maintained at a non-selection voltage level.
FIG. 20 shows an example of driving voltage waveforms indicating a horizontal line at every other scanning-electrode line in the partial display state of this conventional example. A represents waveforms of voltages applied to one pixel oil the upper-half screen, and B represents waveforms of voltages applied to all the pixels on the lower-half screen. In the figure, bold lines in the waveforms A and B indicate scanning electrode driving waveforms, and thin lines indicate signal electrode driving waveforms.
A selection signal V0 (or V5) is sequentially applied to each line of the scanning electrodes for the upper-half screen in every selection period (one horizontal scanning period: 1 H), and a non-selection voltage V4 (or V1) is applied to other lines of the scanning electrodes. ON/OFF information regarding individual pixels oil selected lines is sequentially applied to the signal electrodes synchronously with the horizontal scanning period. More particularly, in a period when application voltages for selected lines of the scanning electrodes are V0, V5 is applied to the signal electrodes of ON-pixels on selected lines and V3 is applied to the signal electrodes of OFF-pixels; in a period when application voltages are V5, V0 is applied to the signal electrodes of ON-pixels, and V2 is applied to the signal electrodes of OFF-pixels. The voltage applied to the liquid crystal for individual pixels is the differential voltage between the scanning voltage applied to the scanning electrode (the selection voltage and the non-selection voltage) and the signal voltage applied to the signal electrode (an ON-voltage and an OFF-voltage). On principle, when this differential voltage is higher, a pixel with a higher effective voltage is turned ON; while, when this differential voltage is lower, a pixel with a lower effective voltage is turned OFF.
On the other hand, as shown in B of FIG. 20, since no selection voltage is applied to the scanning electrode, effective voltages for pixels on the lower-half screen are reduced to be considerably lower than effective voltages applied to the OFF-pixels on the upper-half screen, causing the lower-half screen to be totally in the non-display state.
As shown with a liquid-crystal alternating-current driving signal M, FIG. 20 shows a case in which signal-polarity switching is carried out for a driving voltage in every selection period for 13 lines. In this way, in higher-duty driving for reduction of flickering, cross-talks, and other problems, signal-polarity switching must be carried out for the driving voltages in every selection period for some ten lines. Although the lower-half screen is in the non-display state, voltages applied to the scanning electrodes and the signal electrodes in the non-display region are varied, as shown in B of FIG. 20. In this case, a defect is caused such as that even after the screen turned to be in the partial display state, circuits such as drivers would still continue to operate, and charging and discharging of the liquid crystal would still continue; therefore, power consumption is not expectedly reduced.
For reference, for switching of the display duty, the simple-matrix liquid crystal display apparatus requires modification of setting the driving voltage. This will be described below with reference to FIG. 21, which is an internal circuit of the driving-voltage forming circuit block 54.
First, a description will be given of a construction and functions in FIG. 21. For driving a liquid crystal display panel of a duty higher than about 1/30 duty, voltages of six levels of V0 to V5 are necessary. The highest voltage to be applied to the liquid crystal is V0-V5, and the input power source voltage of V5 is used as it is for V0. By use of a variable resistor RV1 for contrast adjustment and a transistor Q1, the voltage V5 which will result in the suitable contrast is retrieved from an input power sources of 0 V and −24 V. Resistors R1 to R5 are used to divide the voltage V0-V5 for forming intermediate voltages, and operational amplifiers OP1 to OP4 are used to increase driving capacity of the intermediate voltages so as to output V1 to V4. Switches S2a and S2b are interlock switches, and either one of R3a and R3b is connected in series to R2-R4 in accordance with the level of the signal PD. Resistance values of R3a and R3b are differentiated so that V0 to V5 of a different voltage-division ratio can be formed according to the PD level.
Among V0 to V5 there is a relationship expressed by V0-V1=V1-V2=V3-V4=V4-V5, and a voltage division ratio (V0-V1)/(V0-V5) is called a bias ratio. Japanese Examined Patent Publication No. 57-57718 discloses that when the duty is 1/N, a preferable bias ratio is 1/(1+√N). Accordingly, when resistance values of R3a and R3b are set for a 1/400 duty and a 1/200 duty, respectively, driving can be performed at preferable bias ratios.
To switch between duties, not only the bias-ratio switching is necessary, but the driving voltage (V0-V5) must also be modified. If the duty is switched from 1/400 to 1/200 with a fixed driving voltage, even when switching is performed so as to set preferable bias ratio, the display results in being of much lowered contrast. This is caused by the fact that time when selection voltages are added to the liquid crystal is duplicated to excessively increase effective voltages. In the conventional example, while necessity for the bias-ration switching and an implementation means therefor are disclosed in detail, necessity for the driving-voltage switching and an implementation means therefore are not disclosed in detail.
In particular, with a duty assumed to be 1/N, when N>>1, (V0-V5) must be adjusted substantially in proportion to √N. For example, if a preferable (V0-V5) in case of 1/400 duty is 28 V, (V0-V5) must be adjusted to 28V/√2≈20 V in case of 1/200 duty This voltage adjustment is to be carried out by apparatus users by adjusting the contrast-adjustment variable resistor RV1 every time when switching is performed between the full-screen display state and upper-half screen display state. It is very inconvenient for apparatus users. Supplement of a driving-voltage automatic setting means is mandatory; however, it is not so easy as a bias-ratio switching means and the driving-voltage forming circuit will be much complicated. For reference, in the conventional publications, a description is given to the effect that since reduced driving voltages would be sufficient in a half-screen display, power consumption would be further reduced. However, since a large volume of the reduction voltage of 8 V is consumed to allow the contrast-adjustment transistor Q1 to generate heat, the power consumption is not reduced so much.
When the partial display is considerably smaller to cover some ten lines to twenty lines, duty-switching is carried out according to that display. By this, a preferable bias ratio, such as 1/3 and 1/4, can be obtained. In this case, voltage necessary for driving the liquid crystal is not any more the six levels, but will instead be five levels for the 1/4 bias and four levels for the 1/4 levels. When five levels of voltages are necessary, the resistance value at the side to be connected to either one of the resistors R3a and R3b may be set to 0 Ω. However, when four levels of voltages are necessary, the resisters R2 and R4 need to be 0 Ω, not the resisters R3a or R3b. A bias-ratio switching means and a driving-voltage switching means in a case as described above are disclosed in Japanese Unexamined Patent Publication No. 7-281632. However, a further description regarding a construction of the foregoing will be omitted here.
According to the aforementioned methods that have been proposed to date, basic functions for causing partial lines of a liquid crystal display panel to be in a display state and for causing other-lines to be in a non-display state are realized, and power consumption can also be reduced to a certain extent. However, there still remains problems such as that a driving-voltage forming circuit will be very complicated, the number of lines that can be displayed is limited because of hardware, and reduction of power consumption is not yet sufficient.
Furthermore, the former Japanese Unexamined Patent Publication No. 6-95621 is relevant to a transmissive-type liquid crystal display panel, and the latter Japanese Unexamined Patent Publication No. 7-281632 states only about a partial-display method, in which display types are not disclosed. Whatever the transmissive type or reflective type, when higher contrast is considered important, liquid crystal display panels of a normally-black type have been conventionally used. The reasons are described below.
In case of a normally-white type, since regions among dots to which voltage is not applied are in white, white-display regions of a screen appear sufficiently in white, but black-display regions do not appear sufficiently in black. In contrast, In case of the normally-black type, since regions among dots to which voltage is not applied are in black, black-display regions of a screen appear sufficiently in black, but white-display regions do not appear sufficiently in white. Display can be in higher contrast in the case the black-display region appear sufficiently in black than in the case where the white-display regions appear sufficiently in white. For these reasons, use of the normally-black type liquid crystal display panel provides higher contrast.
For reference, the normally-black type is a mode in which a black-display is provided when the effective voltage applied to the liquid crystal is an OFF-voltage which is lower than a threshold of the liquid crystal, and a white-display is provided when the application voltage is increased and an ON-voltage higher than the threshold of the liquid crystal is applied to the liquid crystal. On the other hand, the normally-white type is a mode in which a white-display is provided when the effective voltage applied to the liquid crystal is an OFF-voltage which is lower than a threshold of the liquid crystal, and a black-display is provided when the effective voltage is increased and an ON-voltage higher than the threshold of the liquid crystal is applied to the liquid crystal. For example, when a substantially 90-degree twisted nematic type liquid crystal is used, the liquid crystal display panel has a paired polarizers on two side faces of the liquid crystal display panel; when transmissive axes of the paired polarizers are arranged substantially parallel, the normally-black type is made; when the transmissive axes of the paired polarizers are arranged substantially perpendicular, the normally-white type is made.
FIG. 18 is a drawing illustrating a partial display state in the case when the normally-black type liquid crystal display panel 107 is used. Since the OFF-voltage or the effective voltage lower than the OFF-voltage is applied to the liquid crystal in the non-display region, as shown in the figure, the non-display region provides the black-display. On the other hand, in the reflective type liquid crystal display panel, characters must be displayed in black and the background must be displayed in white so that incident light is reflected to make a bright and easy-to-view display. However, with the normally-black type liquid crystal display panel, while the background of the display region appears in white, the non-display region appears in black. This partial display state is incompatible. Furthermore, with display dots positioned at the border between the display region and the non-display region on the display screen, black-display dots forming characters in the display region and black-display dots in the non-display region become adjacent dots, causing a chained-character display when it is viewed. This gives rise to a problem in that the characters displayed on the dots on the border between the display region and the non-display region are difficult to be identified. For making the non-display region a white display so as not to being incompatible, the ON-voltage needs to be applied to the liquid crystal in the non-display region. On principle, however, such a non-display region cannot be referred to as a real non-display region. If the non-display region is arranged to be the white-display, problems arise such as those described as below. Power consumption by circuits necessary for realizing such an arrangement cannot be reduced. In addition, in a case where liquid crystal molecules are arrayed in the horizontal direction in an OFF-state and are allowed to rise in an ON-state as a nematic liquid crystal, permittivity of liquid crystals in the ON-state is two to three times higher than that in the OFF-state. In this condition, when the liquid crystal is driven to an ON-state so as to display the non-display region in white, charging and discharging current due to AC driving of a liquid crystal layer is increased; in which case, as compared to the case in the full-screen display state, the power consumption in the full-screen display state is not reduced so much, or conversely, is increased.
As described above, when the normally-black type liquid crystal display panel is simply adopted for improvement of contrast, the resulting display is incompatible, because the non-display region is the black-display in the partial display state. Furthermore, if the non-display region is arranged to be the white-display which is not incompatible, it is difficult to refer to such an arrangement as realization of a partial display function when it is viewed on principle, and in addition, an object of power consumption cannot be achieved.